diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_class_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_class_builder.dart
index c143b6d433a4..70a9cd10045f 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_class_builder.dart
@@ -492,7 +492,7 @@ abstract class KernelClassBuilder
         }
       }
 
-      unhandled(
+      return unhandled(
           "${interfaceMember.runtimeType} and ${dispatchTarget.runtimeType}",
           "isValidImplementation",
           interfaceMember.fileOffset,